The present invention relates to circuitry for frequency domain signal measurement in general and more particularly but not exclusively to such circuitry incorporated into intruder detection systems, energy efficiency systems and the like.
There are many kinds of detectors for the purposes of break-in and security, energy control and other purposes, including PIR detectors, microwave detectors, ultrasonic detectors, infrasonic detectors, shock detectors and the like. In all of these detectors, it is the practice to use very sensitive sensors that generally produce a very low signal, which must be amplified in order to allow for the processing of signals.
As is known, one of the main problems in the field of alarm systems is the undesirable, relatively high rate of false alarms. In order to reduce the number of false alarms, there now exist very sophisticated signal processing circuits, sometimes using micro-processors, and these attempt, with the help of sophisticated algorithms and A/D circuits, to better distinguish a real alarm from a false alarm. The accepted solutions are relatively expensive and costly, and the market is very price-sensitive.
A further drawback of sophisticated signal processing is the multiplicity of components which such an approach requires. Every additional component leads to a corresponding reduction in reliability and increases the sensitivity of the system to external RF noise, thus leading to additional false alarms. Reliability of the apparatus and the prevention, or at least reduction, of false alarms are very important issues in the field of security systems.
Passive Infrared Detectors (PIR)
PIR detectors are very popular today in the field of burglar alarm systems and energy control. These detectors use a pyroelectric sensor (explained in U.S. Pat. No. 5,077,549 Col. 1/13-48 and U.S. Pat. No. 5,414,263 Col. 1/12-54, the contents of which are hereby incorporated by reference).
The pyroelectric sensor is connected to a band-pass filter/amplifier having a very high gain of several thousands (generally 5,000). Reference is accordingly made to U.S. Pat. Nos. 4,570,157, 4,468,658, 5,309,147, 4,364,030, 4,318,089, 4,612,442, 4,604,524, the contents of which are hereby incorporated by reference. In these patents the signal is amplified and fed into a window comparator or other voltage comparator, and when the signal exceeds a threshold voltage, the alarm is activated.
In recent years, with the appearance of microprocessors, very sophisticated signal processing methods have been adopted. U.S. Pat. No. 5,077,549, the contents of which are hereby incorporated by reference, describes an alarm based on the principle of signal integration (equivalent to measuring the energy). In this patent it is important to measure the exact form of the signal in order to transform the signal into useful information. In this patent too, use is made of a similar high gain band-pass filter/amplifier.
An additional patent worth noting is U.S. Pat. No. 5,693,943 to Visonic, in which an exact analysis of the form of the signal is used to make a decision regarding a real or false alarm. In this patent too, use is made of high gain amplifiers. Again, it is very important to keep track of the exact form of the signal in order to make the right decision. Similar problems can be seen in U.S. Pat. No. 5,870,022. The contents of both of these patents are hereby incorporated by reference.
In all of the examples cited above and in many others, it can be seen that the electronic circuits contain a large number of components of various kinds, which raise the price of the product and reduce reliability. An additional problem stems from the fact that, due to the weakness of the signal which is generated by the pyroelectric sensor, it is customary to use high-gain amplifiers (between 1,000 and 10,000) and relatively narrow (0.2-8 Hz) pass bands to overcome environmental interference. In these amplifiers, which are low frequency, use is often made of high value capacitors with low leakage. This makes the products more expensive and in particular causes a substantial reduction in reliability, which may be responsible for certain types of false alarm.
The combination of very high amplification and very narrow band pass at low frequency and the use of AC coupling between amplification stages may cause the amplifier to distort the form of the signal. Ringing problems are known as are DC Offset, overshoot and other problems, and these may cause the signal produced by the amplifier to differ significantly from the original signal of the pyroelectric sensor. This, in turn, causes various signal processing problems in detectors which use comparators, and in particular with more sophisticated detectors which analyze the form of the signal (see U.S. Pat. No. 5,084,696, the disclosure of which is hereby incorporated by reference, and U.S. Pat. No. 5,870,022 which is referred to above).
In order to reduce amplifier gain and to improve the processing of the signal, attempts have been made to use high resolution A/D circuits, see for example, U.S. Pat. Nos. 4,546,334 and 5,693,943 referred to above.
Using such a technique, it is possible to reduce some of the levels of amplification which are used in circuits with a window comparator. However, the cost of the circuit rises due to the use of the A/D converter and reliability is not necessarily improved. In recent years, with the appearance of micro-processors comprising internal A/D converters, the use of A/D converters has been expandedxe2x80x94See U.S. Pat. Nos. 5,629,676 and 5,237,330, the contents of which are hereby incorporated by reference. Such use can reduce the required amplification and allows just one amplification stage. However, such processors are more expensive than regular non-A/D processors.
An additional problem, when using PIR detectors, is the question of their immunity to radio frequency interference (RFI) and electromagnetic interference (EMI), which is a main factor in the design of alarm systems with PIR detectors and others. This is a consequence of the low signal levels, and the use of high gain amplifiers with high impedances.
It is also worthwhile noting, regarding PIR detectors, that it is customary to compensate for the effect of the temperature difference between the body of an intruder and room temperature. This can be done directly by means of altering the gain of the amplifier""s analog circuitxe2x80x94see U.S. Pat. Nos. 4,195,234 and 4,943,712, the contents of which are hereby incorporated by reference,xe2x80x94or it can be done more exactly using software and a microprocessorxe2x80x94see U.S. Pat. No. 4,546,344, the contents of which are hereby incorporated by reference, and U.S. Pat. No. 5,629,676, referred to above.
There are PIR detectors of various kinds on the market which use two or more pyroelectric sensors and sophisticated signal processing. These detectors are sometimes called QUAD. See for example patents: EPO198,551, GB 2170952, 4,614938, 4,618,854, 4,704,533, 4,697,081, 4,746,910, 4,912,748, 4,943,800, the contents of which are hereby incorporated by reference.
In the above-mentioned patents each sensor has a separate amplification circuit, such that in practice, the problems discussed above are magnified.
Another kind of detector is a combination of a PIR detector and a detector based on a different technology, such as microwave (MW) or ultrasonic. These are generally called DUAL detectors.
The following patents, EP O147,925, U.S. Pat. Nos. 4,660,024, 4,772,875, 4,833,450, 4,882,567, 5,077,548, 5,216,410, 5,276,427, 5,331,308, the contents of which are again incorporated by reference, show PIR detectors combined mainly with microwave detectors, such that the alarm is activated only when both individual detectors have been activated. All of the above-mentioned patents show PIR detectors which suffer from the above-mentioned problems.
Other Kinds of Detectors
As was explained above with regard to DUAL detectors comprising both PIR and MW, there are detectors which use sensors of various kinds for alarm and other purposes, and there are also combinations of such detectors. For example, in U.S. Pat. No. 3,801,978, the contents of which are hereby incorporated by reference, there is described a combination of MW and ultrasonic detectors.
U.S. Pat. No. 4,401,976, the contents of which are hereby incorporated by reference, shows a combination of ultrasonic, IR and MW detectors. In U.S. Pat. No. 3,573,817, the contents of which are hereby incorporated by reference, there is a combination of several sensors comprising various technologies, for example audio, seismic, electromagnetic and proximity sensors.
U.S. Pat. Nos. 4,991,145, 4,928,085 and 4,920,332, the contents of which are hereby incorporated by reference, show use of acoustic (microphone) detectors for the detection of infrasonic frequencies (changes in air pressure as a result of the opening and closing of doors by a burglar).
In U.S. Pat. No. 4,621,258, the contents of which are hereby incorporated by reference, there is described a detector which operates using the change in capacitance of an antenna and in U.S. Pat. Nos. 5,196,826, 4,970,517 and 4,697,187, the contents of which are hereby incorporated by reference, one can see break-in detectors which operate on the principle of transmission of a microwave signal and an analysis for the presence of the Doppler effect in the reflected signal.
U.S. Pat. Nos. 4,949,075. 4,942,385, 4,016,529, the contents of which are hereby incorporated by reference, show photo-electric detectors which find changes in light beams (in fact generally infra-red), appearing after they have been sent from the light source. These are checked by photoelectric sensors such as Cds, infra-red diodes and others.
U.S. Pat. Nos. 5,047,749, 3,946,224 and 3,803,572, the contents of which are hereby incorporated by reference, show photo-electric detectors having light sensors which detect changes in lighting caused by a burglar moving nearby.
In fire-detection systems, it is customary to use various kinds of temperature sensors, usually thermistors, which check the temperature and changes therein. Here too, the sensors are connected to various amplification and signal processing circuits.
U.S. Pat. Nos. 5,341,122, 5,323,141, 5,192,931, 5,164,703, 4,837,558 and 4,668,941, the contents of which are hereby incorporated by reference, show acoustic detectors which detect the breaking of glass (audio discriminators), operating by means of a microphone or piezoelectric sensor. The signal is processed in various ways and amplified. There are also shock detectors which generally act by means of microphone or piezoelectric sensors and whose purpose is to detect break-in attempts comprising the breaking of a wall, window, door or the like. These detectors are similar to those mentioned above but the signal processing is different.
These detectors also make widespread use of various amplification circuits with problems similar to those discussed above regarding PIR detectors.
In addition to the above mentioned patents, there are numerous patents and detector products in general, and PIR detectors in particular, which use band pass amplifier/filters with very high gain in order to allow signal processing with reasonable reliability as explained above.
There are only a few examples of attempts to avoid the necessity of using amplification circuits of the type described above.
U.S. Pat. No. 4,523,095, the contents of which are hereby incorporated by reference, discloses a system which attempts to avoid use of high-gain amplification. In conventional PIR detector design, an object passing near a detector will produce, as a result of the common design of multiple beam lens systems used in PIR detectors, a relatively high frequency series of pulses, each pulse having a small amplitude. By a manipulation of the fields of view of each detector a method is shown to integrate those small fast pulses into a large measurable pulse. The resultant large pulse can then be analysed by a relatively simple circuit to. indicate an intrusion event.
Another example is to be found in U.S. Pat. No. 4,418,335, the contents of which are hereby incorporated by reference, in which use is made of a charge amplifier instead of the more conventional voltage amplifier. The arrangement allows work to be done directly on the signal produced by the pyroelectric element, without the usual buffering. This is in order to achieve high RFI immunity without complicating the sensor and/or the amplifier and to reduce interference.
The above-mentioned patent makes use of a charge amplifier with a very low input impedance, which significantly reduces interference but creates other problems such as leakage current, which can charge the integration capacitor. The capacitor therefore requires a special discharge circuit.
An additional attempt is described in U.S. Pat. No. 4,929,833. A capacitor is charged to a known voltage and is discharged by means of a current flowing through the pyroelectric sensor. The time from the start of the discharge until a predetermined lower threshold is reached, is measured. This is compared with a nominal discharging time when nothing is detected. If the difference exceeds a given threshold, the alarm is activated. In accordance with the patent, sampling is carried out at a frequency of 8 Hz.
The system described therein has many drawbacks (regarding the method of operation and problems therewith, see U.S. Pat. No. 5,414,263 Col. 1/55-2/18). In addition to the problems therein described, the low 8 Hz frequency makes it difficult to detect signals in the desired range viz. 0.2-15 Hz. For example, a signal with a frequency of 8 Hz may not be detected at all since the effect of the current in the high part of the signal would be cancelled out by the effect of the current in the low part of the signal giving a net effect of 0. In other words, the sensitivity of the detector is very dependent on the frequency of the signal and the detector can be ineffective for certain rates of movement of the intruder/target.
Furthermore, these limitations do not allow sophisticated analysis of a signal as is required of modern intruder detection systems. At best, one can use such a detector as part of a light activation system or lighting control, for example as part of a system for energy management, even that being with the above-described limitations.
An additional, more advanced, attempt can be seen in U.S. Pat. No. 5,414,263, the contents of which are hereby incorporated by reference. As in the previous example, this patent is mainly designed for use in energy and lighting control. In principle, this patent also deals with the measurement of changes in capacitor discharging times, which are proportional to the output current of the pyroelectric sensor.
The system disclosed in the above-mentioned patent, however, still does not meet the precision levels demanded for measurement of the signal (amplitude, time and form), immunity of the measurement to interference, the prevention of false alarms and the ability to distinguish between humans and pets etc., as is required of detectors today.
In U.S. Pat. No. 4,929,823, and in the above-mentioned U.S. Pat. No. 5,414,263, there is disclosed a system using a capacitor, which is attached via an amplifying circuit comprising a transistor, to a pyroelectric sensor, and which discharges at a known rate. The changes in the current of the pyroelectric sensor are amplified by the transistor and cause variations, positive or negative (depending on the direction of the current), in the discharge time of the capacitor.
The capacitor is charged by a signal processing circuit, such as a microprocessor, which, by means of appropriate algorithms, charges the capacitor and then measures the time to discharge to a preset lower threshold. The signal processing circuit checks whether there has been any change in the measured discharge time compared to the xe2x80x9cno detectionxe2x80x9d state discharge time, or whether there is any long term change in the average discharge time. It then decides whether the change is a substantial movement event or not. (see above-mentioned patent Col. 2/40-58.)
The signal processing circuit in U.S. Pat. No. 5,414,263 constantly checks and computes the average discharge time of the capacitor over a long period, to take into account disturbances such as rain and wind. By means of this computation, the threshold level automatically adjusts for signal activity.
The signal processing circuit also includes automatic correction for pyroelectric sensors and various other components in which there are differences in operating specifications and tolerances (see therein col. 2/58-68).
The system described in U.S. Pat. No. 5,414,263 requires a current amplifier, because the current which is produced by the pyroelectric sensor is not sufficient to effect the discharging time of the capacitor in a manner effective for signal processing (see col. 4/27-30 and col. 4/36-39).
In addition, the pyroelectric sensor currents are used to check that the detected movement was in fact substantial, in order to warrant switching (col. 4/33-36).
The signal processing circuit charges the capacitor and measures the time necessary for discharging by means of the transistor until the voltage is equalized. The pyroelectric sensor current can increase or decrease the capacitor discharging time. (col. 4/39-45).
The signal processing circuit compares the capacitor discharge time with the xe2x80x9cnormalxe2x80x9d discharging time or with the average discharge time over a relatively long period in order to determine whether a substantial movement event has occurred. If the time measured is outside the time window around the average discharging time, this is regarded as indicating a substantial movement event. If additional conditions are met, an alarm condition is triggered.
The additional conditions may include a requirement for a minimal number of substantial movement events detected as a cycle, a minimal number of cycles or a specific sequence of events (col. 4/45-55).
By computing the average discharging time over a period, it is possible to dynamically and automatically correct the movement threshold, thus dynamically filtering infra-red interference from the environment and canceling component parameter changes (col. 4/56-col. 5/5).
In one of the embodiments, a microprocessor charges the capacitor via an I/O port to maximum voltage and then allows the capacitor to discharge. The processor measures the capacitor discharging time from the charged voltage level to a second, lower, voltage level.
The time difference between the measured time and the average time (for a long period) serves:
A. To update the average discharge time over a long period.
B. To check whether the detected events are in fact substantial movement events (col. 5/47-67).
The preference is to measure the capacitor discharge 60 times per second and to update the equalizing time 30 times per seconds The update is done by summing the existing average time with the delta of the new discharging time. The preferred ratio is 15/16 of the old average time+1/16 of the last measured time. Thus, the average time can change only at a very low frequency. The detector hence responds only to frequencies which are higher than a particular frequency dictated by the rate of the updates (30 times per second in this embodiment) and the mixing ratio (1/16 in this embodiment). Calculation suggests that, in this embodiment, the lowest frequency to which the detector reacts is 0.4 Hz. (col. 5/68-col. 6/28).
The decision as to whether there is movement is made as follows:
The difference between the measured discharging time and the average equalization time is determined. If the difference is less than the value of the sensitivity threshold which corresponds to the lowest sensitivity which was set, the decision is that there is no movement. If the difference is greater than the sensitivity threshold then the decision is that there is movement.
In order to decide whether or not to activate the detector, it is then necessary to check whether the movement is significant or not. The check is done by counting the number of consecutively occurring movement events. In this embodiment significance is implied by four consecutive events, that is to say if there were only three or less events (consecutively), the counter is reset.
In the preferred embodiment, the rate of sampling is 60 per second and the counter stands at 4 consecutive events. Consequently, four cycles are needed of 60 Hz in order to activate the load. This implies that frequencies greater than 7.5 Hz will not be detected. This is not in fact the case as will be explained below.
The sensitivity threshold and the counter can be chosen as needed. Similarly, a pulse sequence and other times and other algorithms can be chosen.
It is possible to sample the signal 100 times per second and to connect the sample rate (60 Hz) to the network voltage.
U.S. Pat. No. 5,414,263 is known to give rise to the following problems:
1. Sensitivity Threshold (col. 6/36)
Its results are at best similar to the results obtained from conventional detectors which work by comparing the amplified signal voltage to a series of voltage threshold levels in a window comparator, the threshold levels being equivalent to the sensitivity threshold of the measured delays in the above patent.
In other words, what is indicated is whether the measured signal is more or less than the threshold current. In effect, what U.S. Pat. No. 5,414,263 does is to transform a voltage measurement into the time domain using data measured in a conventional manner. The transform into the time domain using a capacitor-based circuit is itself well-known in the art. For example application note DS00513A (1990) published by Microchip Technology Inc. (USA) describes such a transform. A capacitive charging circuit is used to convert an input voltage into time, which can easily be measured using a microcontroller. By use of a CMOS Quad bilateral switch controlled by the microcontroller, the reference voltage is applied. By means of a current converter the circuit provides a linearly variable current as a function of input voltage. The capacitor is charged up until it trips the threshold on the microcontroller I/O input. This generates a software calibration value that is used to calibrate most circuit errors, including inaccuracies in the resistor and capacitor, changes in the input threshold voltage and temperature variations. After the software calibration value is measured, the capacitor is discharged and the input voltage is connected to Vin. The time to trip the threshold is measured for the input voltage and compared to the calibration value to determine the actual input voltage.
In the voltage window comparator system, it is impossible to receive data on the exact value of the signal. Likewise, in the above-mentioned patent one does not measure the exact signal value. In practice, one cannot know whether the signal is 10 times the sensitivity threshold or just greater than it by 2%. Likewise, it is impossible to know whether the signal is 30% of the threshold or 98% of it. Consequently, one cannot measure the form of the signal or process the signals, as is required for example in Visonic U.S. Pat. Nos. 5,693,943 and 5,870,002 or U.S. Pat. No. 5,077,549.
2. The Use of a Counter to Create a Filter in the Field of the High Frequencies
In order to remove interference, it is more important to filter high frequencies than those at or near the detection level. In particular it is important to filter out frequencies associated with the electricity supply system. In many patents in the field this has been achieved by means of filters of various types, mainly analog, most of which are combined with band pass amplifier/filters.
The above-mentioned filters, however, do not perform satisfactorily because their response curve is not sharp enough and therefore they are dependent on the level of the interfering signal voltage. In recent years attempts have been made at more sophisticated digital signal processing which examines inter alia the frequency of the signal and completely filters all frequencies which are outside the desired band. This is done without any dependence on the voltage level. For example, all of the frequencies which are outside the 0.2-15 Hz band would be completely removed regardless of their voltage.
In U.S. Pat. No. 5,414,263, an attempt was made to implement a different kind of filter by means of a specific sample rate (60 Hz), and a counter which counts four consecutive events. Although it is possible to achieve a certain filtering level (in the present case the high frequency which was set was 7.5 Hz), the quality of the filter is worse than conventional analog filters. The following are the problems that may be noted in the system used in U.S. Pat. No. 5,414,263:
A. As with an analog filter, this filter is also dependent on the signal voltage. If the voltage level is high enough in relation to the threshold, the signal may be passed by the filter because there is no synchronization between the interference signal and the sampled frequency.
B. If the interference is a result of frequencies at half of the sampled frequency or at the sample frequency itself or any multiple thereof, the samples might appear at the peak point of the signal or close thereto and then even signals being a fraction of the threshold value would be likely to get through the filter.
C. Inasmuch as the filter works by passing only signals which are larger than the threshold value, a situation may be created wherein there is a set signal on the mains frequency, which is at a lower level than the threshold value (as is in practice), which in effect is not detected or filtered. However, when the real signal appears with a relatively low value which, normally, would not be detected, a situation is created in which the two signals superimposed one upon the other are likely to pass the threshold and to create an unwanted alarm.
D. Inasmuch as several consecutive events are needed to implement the filter action, should there arise interference whilst a movement event is being detected, the superimposed disturbance may ruin the measurement in a specific sample, thereby resetting the event counter without detection. Any attempt to change the manner of the count in order to overcome this could harm the operation of the filter.
E. Despite the fact that the possibility is raised of using a variety of pulse patterns or a minimal number of events or a minimal number of cycles or a particular sequential spread (col4/51-55, col. 6/667-col7/4), it is clear that any such criterion would harm or cancel the actions of the filter in the high range and cause false alarms or non-detection problems.
3. Microprocessor with A/D
In the above-mentioned patent, use is made of a microprocessor which both charges the capacitor and measures the discharge time. Such a microprocessor generally has an I/O with an A/D converter in order to allow it to measure the voltage to which the capacitor discharges. Such a microprocessor is more expensive than a regular processor without A/D. It is possible to use a processor without A/D but the precision of the measurement may be low and it might be easily affected by electrical and other disturbances.
An object of the present invention is to produce PIR and other types of detectors, including combined detectors, with a small number of components, greater signal measurement accuracy, minimal or no amplification, a minimal distortion of the signal, high reliability and better immunity against RFI and EMI interference, and which requires only the most basic microprocessors on the market.
A further object of the present invention is to provide a signal processing circuit for various sensors by means of a very basic. microprocessor, negating the need to use amplification and other circuits and optimally exploiting the characteristics of the microprocessor in order to substantially reduce the price of the product, improve its immunity to disturbance, make the signal processing more precise, with minimal amplification distortion, and enhance the product. According to a first aspect of the present invention there is provided circuitry for frequency domain signal measurement comprising:
a signal input,
a microprocessor, and
an oscillator,
said oscillator being operable to generate a pulse signal, the frequency of which is a function of amplitude of a first signal received at said signal input, and to supply said pulse signal to said microprocessor, and said microprocessor being operable to measure the frequency of said pulse signal by comparing the pulse signal with a timing signal, thereby providing an indication of the amplitude of said first signal.
In an embodiment, the timing signal is in the form of a timing window. Preferably the pulse signal comprises pulses which are countable by a counter, said counter being connected to said microprocessor to give an indication to said microprocessor that a given number of pulses has been counted. Again, preferably the pulse signal is connected directly to said microprocessor. In such a case the pulse signal is advantageously supplied to a clock input of said microprocessor.
The apparatus may further comprise a specially constructed timing circuit, wherein an output of said timing circuit comprises said timing signal. This is helpful when the pulse signal is being connected to the microprocessor""s clock input, as the microprocesser needs an independent timing signal. Preferably the microprocessor is operative to count said pulse signal over said timing window. The clock input may be an external clock input.
In a preferred embodiment, the oscillator is wholly external to said microprocessor. The oscillator may utilize internal features of said microprocessor or it may be wholly external.
The sensor signal may be analog or digital, and the term digital, in this specification, includes not only binary but other discrete level signals.
According to a second aspect of the present invention there is provided circuitry for frequency domain signal measurement comprising:
a signal input, a microprocessor and a clock oscillator circuit operable to generate a clock signal for said microprocessor, wherein the frequency of pulses of said microprocessor clock signal is variable as a function of the amplitude of a signal received at said signal input, and
said microprocessor is operable to process the clock signal and to provide an output indication of the amplitude of said signal received at said signal input.
The circuitry preferably comprises a timer operable to define a pulse counting time duration for counting a plurality of said clock pulses, the timer being further usable by said microprocessor in processing said signal. Preferably the microprocessor is operable to count a plurality of pulses over said time duration. The timer may comprise a capacitor-based circuit and may additionally be connectable to utilize an I/O port of said microprocessor. Preferably the clock oscillator circuit utilizes a microprocessor built-in clock circuit, but may alternatively be wholly external to said microprocessor.
The signal received at the sensor input may be analog or digital, and, as mentioned above, the term xe2x80x9cdigitalxe2x80x9d covers not only binary but also other types of discrete level signal.
The signal received at the signal input is preferably from one or more sensors, which may be part of a security system and may be one of a whole series of sensors including an infra-red sensor, and a pyroelectric sensor. The sensor may be connected to said clock oscillator circuit via an interface circuit, which may be operable to perform buffering or even amplification.
According to a third aspect of the present invention there is provided detection apparatus comprising a sensor providing sensor signal output, a microprocessor, and a clock oscillator circuit generating a clock signal for said microprocessor, wherein the frequency of said microprocessor clock signal varies as a function of the amplitude of said sensor signal, and said microprocessor processes the clock signal and provides a detection indication when said sensor signal fulfils certain criteria. Preferably such apparatus further comprises a timer operable to define a pulse counting interval for counting a plurality of said clock pulses, and wherein said timer is usable by said microprocessor in processing the signal.
The microprocessor is preferably operable to count said plurality of pulses over said time duration. The timer preferably comprises a capacitor-based circuit, and utilizes an I/O port of said microprocessor. The clock oscillator may be external to said microprocessor but may utilize the microprocessor built-in clock circuit.
The sensor signal may be analog or digital as mentioned above.
The circuits discussed above are useful for, inter alia, intrusion prevention, theft prevention, lighting control, vibration sensing, shock sensing, and displacement sensing.
Preferably the sensor is any one of a group comprising an infra-red sensor, a quad-element infrared sensor, an acoustic sensor, an infrasonic sensor, an ultrasonic sensor, a photoelectric sensor, an electromagnetic field sensor, a temperature sensor, and a smoke-detecting sensor.
In an embodiment, there is provided a second sensor, which may be any one of a group comprising an infra-red sensor, a quad-element infrared sensor, an acoustic sensor, an infrasonic sensor, an ultrasonic sensor, a photoelectric sensor, an electromagnetic field sensor, a temperature sensor, and a smoke-detecting sensor. The microprocessor may process the two signals from the two sensors either by time multiplexing (e.g. connecting to one and then the other) or by distinguishing between the two based on the characteristics of the signals (e.g. frequency). As will be apparent to persons skilled in the art, a plurality of sensors may be incorporated into a single apparatus.
According to a fourth aspect of the present invention there is provided a method for signal measurement comprising:
providing a first signal to an oscillator circuit operable to generate a clock signal for a microprocessor, wherein the frequency of said clock signal is variable as a function of the amplitude of said first signal, and said microprocessor is operable to process the clock signal and to provide an output indication of the amplitude of said first signal.
An embodiment allows for defining a pulse counting time duration for measuring of said first signal.
Preferably, said analog signal generates clock pulses of said microprocessor clock and said microprocessor is operative for pulse counting of said clock pulses over said time duration. Alternatively, the analog signal generates clock pulses of said microprocessor clock, and said microprocessor counts pulses having a frequency which is a function of the frequency of said clock pulses over said time duration.
Preferably the step of measuring the modulation of said frequency comprises applying the modulated frequency to the external clock input of a microprocessor to produce clock pulses, applying a windowing signal to said microprocessor to define a measurement window, and counting a number of clock pulses occurring within said measurement window. The weak signal source may typically be an intrusion sensor, for example a pyroelectric sensor.
An embodiment of the method comprises the additional steps of placing a calibration radiation source in association with said intrusion sensor,
applying a measured amount of energy to said calibration radiation source to cause said calibration radiation source to produce radiation,
measuring an output of said sensor produced in response to said radiation, and
calculating a correction factor to cancel out any deviation of said output from an expected output.
In a further aspect of the present invention there is provided circuitry for signal measurement comprising an input for receiving a signal having a varying amplitude from a sensor, a converter for converting said varying amplitude into a varying frequency, and a measuring device operable to determine parameters of the sensor signal by measurement of variations in the frequency.